IMHO - all the catback does is make the Jeep sound better. As far as HP boost or mileage boost I believe it's negligible. (Considering Tires, gears, Lift)
I had the Flowmaster 40 on my '02. Sounded great. Other benefit I found was that it was tough as nails. Everything on the undercarriage took a beating when I lived in Phoenix and played with rocks.
Mine was about $140 installed. Also, don't put tips on...they'll snag or get bent off, trust me.